Research On Glass-Coatings On BaTiO₃-Based PTC Thermistors

With the rapid development of microelectronics, the traditional single-layer electronic component can no longer satisfy the need of the market. Instead it is a necessary trend for the integration and chip type of the electronic components. The multilayer chip type PTC ceramic is widely used due to its superior performance. Chemical plating method is needed in the electrode manufacture of the multilayer chip type PTC ceramic component. During the process, plating solutions would permeate into ceramic bodies and metals can be formed on the ceramic areas without terminal electrodes due to the semiconductivity of PTC ceramics. This can lead to the short cut between the terminal electrodes. In addition, when applied into actual environment, the PTC ceramic component could be eroded by the moisture. To solve these problems, it is indispensable to apply a protective coating material on the PTC ceramic component. Glass is one kind of ideal sealing materials. It is meaningful and important to develop lead-free sealing glass.Borosilicate glass system has great advantages in chemical and thermal stability, non-crystallization and lead-free aspects. Na2O-K2O-K2SiF6-ZnO-Al2O3-B2O3-SiO2 system was formulated by adding a certain amount of other oxides and fluoride on the base of borosilicate glass system. This glass system can be used to protect Ba TiO3-based PTC thermistors as an alternative to Pb-based glass. All glass samples were prepared by melting-quenching method, and the as-mentioned glass was made into glass organic slurries and sprayed onto the PTC thermistors to complete the sealing process.The research on the composition of the glass organic slurry shows that when the weight ratio of LS binder, dispersant, acetone and glass powder is 0.267: 0.009: 1: 1, the slurry possesses the optimum properties and is desirable for spraying technique.In this paper, the effects of glass formulations on the properties of the glass system and the influence of glass coatings on the electrical properties of PTC thermistors were discussed. The study shows that SiO2/R2 O ratio(in wt %, R=Na, K) exerts the main effect on the glass properties. When the ratio is high(greater than 2.7), the glass layer after sintering is transparent and amorphous; when the ratio decreases below 2.7, there is great chance for the glass layer to crystallize. As the ratio decreases, the fluidity of the glass system increases while the maximum shrinkage temperature TMS and inflection temperature TINF decrease. If the sintering temperature of the glass is higher than 750 °C, the room temperature resistance of PTC thermistors with glass layer increases obviously; when the sintering temperature is less than 750 °C, the room temperature resistance of PTC thermistors with glass layers is steady and almost the same with that of PTC thermistors without glass layers. Besides, the glass layers have little influence on the resistivity jump and PTCR effect of PTC thermistors.